Field of the Invention
The present invention relates to improvements of a radio-frequency device configured to detect a direction toward a desired communication object, according to a signal received from the communication object, and a radio-frequency tag communication device configured to effect radio communication with radio-frequency tags for writing and reading information on and from the radio-frequency tags.
Description of Related Art
As an example of the radio-frequency device for effecting communication with a desired communication object, there is known an RFID (Radio-Frequency Identification) communication system wherein a radio-frequency tag communication device (interrogator) reads out information, in a non-contact fashion, from small-sized radio-frequency tags (transponders) on which desired information is written. In this RFID communication system, the radio-frequency tag communication device is capable of reading out the information from the radio-frequency tags, even where the radio-frequency tags are contaminated or located at positions invisible from the radio-frequency tag communication device. For this reason, the RFID communication system is expected to be used in various fields, such as management and inspection of articles of commodity.
As one form of application of such a radio-frequency tag communication device, there is known a technique to change the directivity of communication between the communication object in the form of the radio-frequency tag and the radio-frequency tag communication device, for thereby detecting the direction toward the radio-frequency tag. For instance, Patent Document 1 discloses a method of reception of a radio-frequency signal, in which the angle of directivity of a directivity-variable antenna device is controlled. In this method, the angular range of directivity of the directivity-variable antenna device is controlled on the basis of a ratio of a desired wave potential and an interference wave potential of the signal received by the directivity-variable antenna device, for suitably detecting the direction toward the communication object in the form of the radio-frequency tag.
As one form of the radio-frequency tag communication device, there is proposed a portable radio-frequency tag communication device (handy scanner) which is movably provided in a desired area of detection such as a room. For example, Patent Document 2 discloses a portable terminal, which is a light-weight and small-sized radio-frequency tag communication device which is easy for the user to carry.
In the above-described radio-frequency tag communication device, the directivity of communication is usually controlled to improve the sensitivity of communication. Various techniques are used to suitably control the directivity of communication. For example, Patent Document 3 discloses a directivity control technique for a radar device. According to this technique, which applies to an array antenna device provided with a plurality of antenna elements and capable of a phased array control, the antenna elements are selectively used for communication, to thereby change distances between the adjacent antenna elements, for thereby controlling the directivity characteristic of the array antenna device, so that the sensitivity of reception is suitably matched over the entire area of detection, without lowering the maximum communication distance and sensitivity.
Patent Document 1: JP-2003-243921 A
Patent Document 2: JP-2001-307032 A
Patent Document 3: JP-6-174823 A
Although the conventional technique is advantageous for a reduced amount of calculation in a generally known operation of BFA (Beam Forming Antenna) processing for the directional detection, the conventional technique suffers from a drawback of a low degree of resolution and consequent incapability to accurately detect the direction toward the desired communication object in the form of the radio-frequency tag. On the other hand, there are known other techniques such as MUSIC process and ESPRIT process capable of the directional detection with a high degree of resolution. However, these techniques undesirably requires complicated processing and a large amount of calculation. Namely, there has not yet been developed a technique for the directional detection with a high degree of resolution and with a simple processing operation.
For maximizing the maximum distance of communication of the conventional portable radio-frequency tag communication device, it is considered to control the main lobe direction of a directivity-variable antenna device. However, the state of communication may vary with a change of attitude of the portable radio-frequency tag communication device, for example, when the device is inclined by the user. Accordingly, the conventional portable radio-frequency tag communication device suffers from instability to maintain its optimum state of communication. In this respect, there has been a need for developing a radio-frequency tag communication device capable of optimally controlling the directivity of communication irrespective of the attitude of the device.
In the directivity control of the array antenna device provided with the plurality of antenna elements described above, it is required to increase the number of the antenna elements, for reducing the width of the main lobe to thereby narrow an effective angular range of the directivity of communication, so that the device undesirably tends to be large-sized. Thus, there has been a need for developing a radio-frequency tag communication device which is simple in construction and which has a comparatively high level of directivity of communication.